Large-scale continuous rice cooking apparatus for industrial purposes are mainly divided into two types below (as described in "Rice Cooking Techniques and Their Applications", edited by Hiroaki Horigome, Kogyo Gijutsukai K.K., pp. 314-347 (1990)).
(1) Gas Type Continuous Rice Cooking Machine: A chain conveyor for moving rice cookers is installed to run over a fire grate, and rice is cooked by passing the rice cookers over the fire grate successively. In this method, heating conditions in three stages, i.e., boiling, steaming and baking, which have been utilized traditionally in rice cooking, are reproduced.
(2) Steam Type Conveyor Rice Cooking Machine: Rice is put on a conveyor and moved through a steam tunnel for cooking.
Rice is conveyed so as to successively slide through three steps, i.e., primary steaming, hot-water immersion (water absorption) and secondary steaming, whereby the rice is cooked continuously with movement of the conveyor.
The conveyor rice cooking machine of the above (2) is advantageous in that a rice cooking capability is more flexible and the machine is more easily adapted for larger-scale production, because the amount of rice can be optionally determined depending on the width of the conveyor and the height of rice put on the conveyor.
In addition, other various proposals have also been made for the purposes of, for example, improving quality of cooked rice to become more close to the ideal obtained by using rice cookers, achieving adaptability to various kinds of raw rice and a variety of finally cooked rice, and saving a space required for installation. Typical examples of those proposals are below.
(2-1) "Apparatus for Steaming Grains" (Japanese Unexamined Patent Publication No. Sho 63-9406), "Grain Steaming Apparatus" (Japanese Unexamined Patent Publication No. Sho 63-245646), "Grain Processing Apparatus" (Japanese Unexamined Patent Publication No. Hei 4-129510), and "Grain Continuous Processing Apparatus" (Japanese Unexamined Patent Publication No. Hei 5-308915).
These apparatus each comprises a combination of a primary steaming unit for heating rice by steam, an immersing unit in which the rice heated by heat is passed through hot water to absorb water, and a secondary steaming unit for promoting change into .alpha.-starch of the rice having absorbed water, these units being arranged in the order named. This type of continuous steaming apparatus has a feature that an installation space can be saved while ensuring a relatively high production capability. A series of the above-cited Patent Publications disclose various improvements in the component units and combinations thereof. The disclosed apparatus are however complicated in structure as a whole because they need a combination of three units having different specific roles, i.e., the primary steaming unit, the immersing unit and the secondary steaming unit.
(2-2) "Method for Steaming Hard Grains" (Japanese Unexamined Patent Publication No. Sho 59-11150).
This Patent Publication discloses a method for steaming hard grains, which comprises the steps of placing grains on a net conveyor in the form of a uniform layer, heating the grain layer by steam to effect primary steaming, sprinkling water to the grain layer while the grain layer is agitated, causing the grains to absorb water, and heating the grain layer again by steam as with the primary steaming, thereby effecting main secondary steaming. The disclosed method is featured in that an immersing unit such as described in the above (2-1) is not used, grains are continuously processed while being held on a single conveyor, and the amount of water sprinkled is adjusted depending on the property of grains, allowing the grains to absorb water properly. Hitherto, it has been generally considered difficult to absorb water into rice uniformly by the method of sprinkling water. According to the description of this Patent Publication, uniform water absorption by rice can be achieved by sprinkling water under agitation of the rice layer and controlling the amount of water sprinkled. However, the disclosed method is disadvantageous in that an agitating device must be disposed above the belt conveyor and grains of rice in a brittle condition before change into .alpha.-starch are easy to collapse, because the rice is agitated before the step of absorbing water into the rice having been subjected to the primary steaming or immediately after the start of the water absorbing step.
(2-3) "Method and Apparatus for Steaming Grains" (Japanese Unexamined Patent Publication No. Hei 9-107896) and "Method and Apparatus for Steaming Rice" (Japanese Unexamined Patent Publication No. Hei 9-294678)
These Patent Publications disclose a continuous rice steaming line for cooking rice carried on a belt conveyor through a primary steaming step of heating the rice by steam, a water applying step of sprinkling water to the rice, and a secondary steaming step of heating the rice by steam. The primary steaming step and the secondary steaming step employ belt conveyors running at different levels independently of each other.
The disclosed apparatus are featured in that water is absorbed into rice by sprinkling the water without using an immersing unit such as described in the above (2-1). In spite of not using any immersing unit, the disclosed apparatus are still complicated in structure because of the need of two belt conveyors which are disposed at different levels for the primary steaming and the secondary steaming.